The config package used `package _config` with a leading underscore,
which is unconventional in Go. Rename to `package config` (matching the
directory name) and use `cfg` as the standard import alias across all
93 importing files.
Replace all fmt.Printf/Println and log.Printf/Fatal with structured
zap.Logger calls to eliminate inconsistent logging (anti-pattern #12).
- network/crypt_conn: inject logger via NewCryptConn, replace 6 fmt calls
- signserver/session: use existing s.logger for debug packet dumps
- entranceserver: use s.logger for inbound/outbound debug logging
- api/utils: accept logger param in verifyPath, replace fmt.Println
- api/endpoints: use s.logger for screenshot path diagnostics
- config: replace log.Fatal with error return in getOutboundIP4
- deltacomp: replace log.Printf with zap.L() global logger
Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.
